Chaos will out: Bigger, more terrible storms, increasing in severity, frequency; areas of serious weather danger like Tornado Alley and Andra Pradesh increasingly uninhabitable; chaotic weather pushes the world into permanent disaster.
Linear projection: Increase in temperature continues or increases; fight against desertification becomes a failing rearguard action; forests brown, plains become dustbowls; world collapses into food crisis, forced migration and species extinctions.
Tidy correction: An environmental effect (e.g. melting ice dumping into ocean currents) triggers a mini ice age, giving us the chance to experiment with frost-resistant crops, wear fashionable mittens and reintroduce the wolf to N. Europe.
Sudden collapse: The world gets suddenly hotter; increase in carbon dioxide and water vapour triggers a positive feedback loop; after a very brief interval to wonder what went wrong a runaway greenhouse effect extinguishes all life on earth.
Normalisation: Our climate becomes more stable as the planet gradually cools over time; there will be no more ice ages; over the millenia, weather will become more stable and boring, until it finally loses momentum and stops altogether.